Co-injection nozzles or hot runner co-injection devices for injection moulding apparatus, with which two different melts can be simultaneously injected through a nozzle orifice into a moulding chamber or cavity of an injection mould, have been known for a long time (for example U.S. Pat. No. 4,657,496). Most older co-injection nozzles have two separate channels for the two melts, which are disposed in a manner such that a two-layered stream of melt is discharged from the nozzle orifice.
For the production of multilayer injection moulded products, in particular protective containers for foodstuffs, pharmaceutical products, blood samples, etc., with what is known as a barrier or sealing layer, a special type of co-injection nozzle is actually used in which the outflow stream is triple-layered and concentrically configured, wherein the barrier layer forms the middle layer.
WO 81/00231 discloses a co-injection nozzle of this type, which combines three separate melt streams in one triple-layered, concentric melt outflow stream. In that nozzle, the inner melt stream can be regulated using a valve needle disposed in a central bore of the nozzle.
In other co-injection nozzles of this basic type, a first melt is divided into two streams outside or inside the co-injection nozzle which then form an inner and outer layer of the concentric outflow stream. A second melt is guided between the two layers and forms the middle barrier layer. The three layers are then combined into a multilayer melt stream outside or inside the co-injection nozzle and then injected into the mould cavity as a concentric outflow stream, whereupon a multilayer injection moulded product is formed with a barrier layer that is covered on both sides. The melts of the various layers can be regulated as a function of the type of embodiment of the co-injection nozzle or the co-injection device. In order to enclose the barrier layer completely in the melt for the outer and inner layer, at the respective start and end of an injection moulding procedure, only the melt for the outer and/or the inner layer is injected, without the melt for the middle layer.
EP 0 929 390 discloses a co-injection nozzle in which the three melt layers are combined in a combination unit disposed upstream of the nozzle and then guided along an elongate tubular flow channel to the nozzle orifice. The tubular flow channel is formed by a central bore in the nozzle body and a valve needle disposed therein. The valve needle can be used to adjust the flow of the inner melt layer in the combination unit. In addition, the flow of the individual melt streams is regulated via the supply unit.
EP 0 911 134 describes a co-injection unit in which three melt streams are guided through a respective melt supply opening into the co-injection nozzle and are combined to form a concentrically layered melt stream in the nozzle tip region shortly before the nozzle orifice. The melt for the inner layer is guided in an annular inner melt channel which is formed by a central bore and a valve needle. The melt for the middle layer is guided in an annular middle melt channel which extends about the annular inner melt channel. The melt for the outer layer is guided in an annular outer melt channel which extends about the annular middle melt channel. The inner and middle melt channels can be closed off by the valve needle while the outer melt channel remains open.
WO 00/54955 discloses a co-injection nozzle in which the two melts for the inner and middle layer are combined in a first upstream combination unit outside the co-injection nozzle and then guided together along an inner central melt channel to the nozzle orifice in order to obtain a combined melt stream which is as stable as possible. In the region of the nozzle tip, the melt for the outer layer is combined with the already combined central melt stream and then injected like this into the mould cavity.
WO 04/103668 discloses a co-injection device in which a first melt stream is divided within a co-injection nozzle into two streams for the inner and outer layer. The divided streams are combined in a combination chamber with the second melt for the middle layer upstream of an elongate central melt channel in order to form a concentrically layered melt stream which then is guided via the central melt channel along a valve needle to the nozzle orifice. The combination chamber is thus configured in a manner such that the formation of the middle layer can be regulated with a minimum amount of material from the two streams of the first melt, avoiding instabilities in the flow.
EP 2 054 209 discloses a co-injection device in which a first melt is divided into two streams upstream of the inlet into the co-injection nozzle. The divided streams are then merged with the second melt in the region of the nozzle tip in order to form a multilayer melt stream.
WO 11/006999 describes a co-injection device in which two melts are supplied laterally of a co-injection nozzle, wherein the first melt is divided within the co-injection nozzle into a stream for the inner and outer layer respectively. The streams are combined in the nozzle tip. The co-injection nozzle has a movable needle and a movable sleeve to regulate the individual melt streams.
WO 12/037682 discloses a co-injection nozzle in which a portion of a first melt stream is guided through an annular second melt stream via lateral tunnel channels. The three melt streams are combined in the region of the tip to form a multilayer melt stream. The inflow of the middle melt stream can be regulated with a movable sleeve.
The material for the barrier layer is expensive, and so in multilayer injection moulded products, it is preferably present in a layer which is as thin as possible. Furthermore, at the start and end of the respective injection moulding cycle, only the first melt is injected and the melt stream for the second melt, which forms the barrier layer, is interrupted in order to obtain an injection moulded product with a completely encapsulated barrier layer. Precise regulation of the second melt is thus desirable in order to produce injection moulded products with very thin barrier layers.
One problem which can occur with known co-injection nozzles, however, is back-flow of the second melt in the middle melt channel. If a back-flow of the second melt of this type occurs, this results in an inaccurate supply of the second melt in the next injection moulding cycle, and thus in inaccurate or defective barrier layers for the injection moulded products.
In the co-injection nozzles of WO 11/006999 and WO 12/037682, back-flow of this type can be prevented by means of a movable sleeve which can close off the annular middle melt channel. Furthermore, the construction of a co-injection nozzle of this type and of the co-injection device is difficult and expensive because of the additional movable parts in the co-injection nozzle.
Other co-injection devices, such as those known from WO 00/54955 or EP 0 901 896, have a back-flow control valve which is disposed outside the co-injection nozzle. EP 0 901 896 in fact concerns a co-injection nozzle with a concentric melt outflow stream with only two layers, wherein back-flow is not so serious, because it is not suitable for the production of injection moulded products with a barrier layer. In WO 00/54955, the back-flow control valve is disposed upstream of the co-injection nozzle in a combination unit between a front melt manifold plate for the first melt and a rear melt manifold plate for the second melt.
The known co-injection nozzles with back-flow control valves—whether they are controlled via a movable sleeve or via an upstream back-flow control valve—are of complex, multi-part construction, which is reflected in the high production and maintenance costs.
In all known co-injection nozzles with triple-layered and concentrically configured outflow streams, division of the first melt and combination of the melts to form a layered stream takes place at least in part outside the co-injection nozzle, or they have a multi-part construction with many complex major components. This is particularly the case when, in addition, a back-flow control valve is provided for the second melt.